STRUCTURE, SENSORY EQUIPMENT, AND SECRETIONS OF THE OVIPOSITOR IN A GIANT SPECIES OF HYMENOPTERA - MEGARHYSSA-ATRATA F (ICHNEUMONIDAE, PIMPLINAE)

Megarhyssa atrata F., the largest hymenopteran parasitoid known, is fo und in the holarctic. The stylus of its ovipositor can reach a length of 142 mm, the longest of all known genitalia in Arthropoda. This pimp line parasitizes its host, Tremex columba L., at a great depth in wood . Valvulae 3, which are always shorter than the stylus, are characteri zed on their outer side by abundant trichoid sensilla of three differe nt types, and on their inner side by a dense layer of cuticular spines probably used in cleaning and shielding the stylus while at rest. The joint region between stylus and abdominal segments is richly equipped with mechanoreceptive trichoid and campaniform sensilla. The oviposit or canal is located between the two valvulae 1 which are separate for their entire length and supported proximally by the two valvulae 2 whi ch are joined to each other from the base. The distal end of the stylu s, shaped like an indented lancet, has secretory pores, the first such to be observed in Hymenoptera, from which accessory gland secretions flow. These secretions reach the end of the stylus by intravalvular ca nals in valvulae 1 and 2. These substances, which may cause cell lysis and also act as lubricants, can accumulate among the tens of thousand s of cuticular formations distributed along the stylus which are thoug ht to be surface storage cavities for the secretions. The sensory equi pment of the stylus consists of six rows of approximately 200 mechanor eceptive campaniform sensilla. The ovipositor of M. atrata is thus cha racterized by almost exclusively mechanoreceptive sensory equipment; c hemical stimuli appear to be monitored by two types of trichoid sensil la on valvulae 3. The finding that secretory pores occupy exactly the same sites as chemoreceptors in Ichneumonidae, Chalcidoidea, and Proct otrupoidea demonstrates the importance of studying the ovipositor of t his parasitoid adapted to withstanding extreme pressures as it penetra tes deep into wood.